Search the posts in the ZX6R forums. There are 2 companies that make Big Bore kits for the 636's. Wiseco is one and there is another. It will make it to 662cc's but you will have to re-plate the cylinders. http://www.dynoman.net/engine/pistons/kawa.html
I think JE makes piston kits also for the 636.

Get with these guys. They will build you whatever you want. Falicon cranks I didn't see any 636 parts, but these cats are about the best around. I've built stroker Hondas, and Kawis from their parts, and still have the Honda, and it still runs without issue.